Occurrence of quinolone resistance inStaphylococcus aureusfrom nosocomial infection

Abstract

SUMMARYAmong 63Staphylococcus aureusisolates (one isolate per one patient) counted from infections (from August to November 1991) in hospital T., eight exhibited resistance to fluoroquinolones. Seven of these quinolone-resistant isolates were multiply- and methicillin-resistantS. aureus(QR-MRSA). The results of phage-, plasmid- and genotyping (pulsed field electrophoresis) revealed that six different strain-clones of these MRSA were spread in the hospital.In vitrospontaneous mutants resistant to fluoroquinolones are 1–-100-fold more frequent in MRSA than in otherS. aureuswhen selected on isosensitest-agar containing 1 μg/ml of ciprofloxacin. However, the same mutant frequencies were found in strain 8325–4 with and without the mecA-determinant. The resistance phenotype was stable over 30 generations of subculture in nutrient broth as well in natural quinolone resistant MRSA as in mutants of other types ofS.aureusselectedin vitro. The phenotypic association of quinolone resistance and MRSA is rather likely due to a higher frequency of spontaneous resistant mutants which are present in natural populations of MRSA. Data of chemotherapy prior to the isolation of 8. aureus show that three of seven patients from whom QR-MRSA were isolated were treated with a quinolone. In eight cases of infections with non-MRSA and quinolone treatment the isolatedS. aureusstrains werein vitrosensitive to quinolones.